For many years, exploration and treatment of various organs or vessels has been possible using catheter-based diagnostic and treatment systems. Such catheters are introduced through a vessel leading to the cavity of the organ to be explored or treated or alternatively may be introduced directly through an incision made in the wall of the organ. In this manner, the patient avoids the trauma and extended recuperation times typically associated with open surgical procedures.
To provide effective diagnosis or therapy, it is frequently necessary to first map the zone to be treated with great precision. Such mapping may be performed, for example, when it is desired to selectively ablate current pathways within a heart to treat atrial fibrillation. Often, the mapping procedure is complicated by difficulties in locating the zone(s) to be treated due to periodic movement of the heart throughout the cardiac cycle.
Previously-known systems for mapping the interior of a vessel or organ are described, for example, in U.S. Pat. Nos. 6,546,271 and 6,226,542. The catheters described in those patents employ electromagnetic, electrical, magnetic or acoustic sensors to map the position of a distal end of the catheter in space and then construct a three-dimensional visualization of the vessel or organ interior.
One drawback of such previously known mapping systems is that they rely on manual feedback of the catheter and/or impedance measurements to determine when the catheter is properly positioned in the vessel or organ. Those systems do not measure contact forces with the vessel or organ wall or detect contact forces applied by the catheter against the organ or vessel wall that may modify the true wall location. Instead, previously known mapping methods are time-consuming, dependent upon the skill of the clinician, and cannot compensate for artifacts created by excessive contact forces.
It therefore would be desirable to provide apparatus and methods for detecting and monitoring contact forces between a mapping catheter and the wall of the organ or vessel to permit faster and more accurate mapping. It also would be desirable to provide apparatus and methods that permit the process to be automated.
Once the topography of the vessel or organ is mapped, either the same or a different catheter may be employed to effect treatment. Depending upon the specific treatment to be applied to the vessel or organ, the catheter may comprise any of a number of end effectors, such as but not limited to RF ablation electrodes, rotary or scissor action cutting heads, laser ablation system, injection or sewing needles, fluid conveyance systems, forceps, manipulators, mapping electrodes, endoscopic vision systems and therapeutic delivery systems such as genetic impregnation devices. Exemplary systems are described, for example, in U.S. Pat. Nos. 6,120,520, 6,102,926, 5,575,787, 5,409,000 and 5,423,807.
The effectiveness of such end effectors often depends on having the end effector in contact with the tissue of the wall of the organ or vessel. Many previously-known treatment systems include expandable baskets or hooks that stabilize the distal extremity of the catheter in contact with the tissue. Such arrangements, however, may be inherently imprecise due to the motion of the organ or vessel. Moreover, the previously-known systems do not provide the ability to sense the load applied to the distal extremity of the catheter by movement of the tissue wall.
For example, in the case of a cardiac ablation system, at one extreme the creation of a gap between the end effector of the treatment system and the tissue wall may render the treatment ineffective, and inadequately ablate the tissue zone. At the other extreme, if the end effector of the catheter contacts the tissue wall with excessive force, it may inadvertently puncture the tissue, resulting in cardiac tamponade.
In view of the foregoing, it would be desirable to provide a catheter-based diagnostic or treatment system that permits sensing of the load applied to the distal extremity of the catheter, including periodic loads arising from movement of the organ or tissue. It further would be desirable to have a load sensing system coupled to control operation of the end effector, so that the end effector is operated, either manually or automatically, only when the contact force is detected to fall within a predetermined range.
U.S. Pat. No. 6,695,808 proposes several solutions to measure the force vector arising from contact with the tissue surface, including mechanical, capacitive, inductive and resistive pressure sensing devices. One drawback of such devices, however, is that they are relatively complex and must be sealed to prevent blood or other liquids from disturbing the measurements. In addition, such load sensing devices may result in an increase in the insertion profile of the distal extremity of the catheter. Still further, sensors of the types described in that patent may be subject to electromagnetic interference.
One previously-known solution for dealing with potential electromagnetic interference in the medical environment is to use light-based systems rather than electrical measurement systems. One such light-based system is described in U.S. Pat. No. 6,470,205 to Bosselman which describes a robotic system for performing surgery comprising a series of rigid links coupled by articulated joints. A plurality of Bragg gratings are disposed at the articulated joints so that the bend angle of each joint may be determined optically, for example, by measuring the change in the wavelength of light reflected by the Bragg gratings using an interferometer.
International Publication No. WO 01/33165 to Bucholtz describes an alternative spatial orientation system wherein wavelength changes measured in a triad of optical fiber strain sensors are used to compute the spatial orientation of a catheter or other medical instrument. Although Bucholtz discloses that the strain sensors may be encased within a deformable sheath, as is also described in Bosselman, calculation of the bend angles is not described as requiring characterization of the material properties of the deformable sheath.
Accordingly, it would be desirable to provide diagnostic and treatment apparatus, such as a catheter or guide wire, that permits sensing of loads applied to a distal extremity of the apparatus, but which do not substantially increase the insertion profile of the apparatus. It is further desirable to provide diagnostic and treatment apparatus, such as a catheter and guide wire, that permits computation of forces applied to a distal extremity of the apparatus, and which are substantially immune to electromagnetic interference.
An article by J. Peirs et al., entitled “Design of an Optical Force Sensor for Force Feedback during Minimally Invasive Robotic Surgery,” published by Katholieke Universiteit Leuven, Belgium, describes a tri-axial force sensor for use generating force feedback systems in a robotic surgery system. The apparatus includes a plurality of optical fibers that direct light onto a mirrored surface disposed adjacent to a distal tip of the device. The intensity of the light reflected from the mirrored surface is measured and may be correlated to the force required to impose a predetermined amount of flexure to the distal tip. The article describes a flexible and compact structure that may be used to produce variations in light intensity responsive to contact forces that deform the structure.
International Publication No. WO 2007/015139 to Leo, et al. (Leo), discloses a device and method for resolving a force vector (magnitude and direction) applied to the distal end of a catheter. Leo discloses the use of fiber optic strain elements in a catheter that maintains essentially the same profile as with catheters that do not sense touching forces and is substantially immune to electromagnetic interference.
Temperature changes to the probe of Leo may cause the strain elements to expand or contract relative to a reference or null state and present a false indication of mechanical strain on the elements. Accordingly, Leo discloses the use of additional fiber optic strain elements that are not subject to a mechanical load to provide an indication of the effect of the bulk temperature change of the probe, such as the shift in the returned wavelength of a fiber Bragg grating. Leo also discloses the use of temperature sensors disposed in the distal end of the probe to enable compensation of deformations arising from temperature variations.
While the catheter with fiber optic strain elements in Leo may be useful in certain medical procedures, a limitation to the temperature indication techniques disclosed above is that the sensors are influenced by a temperature at a location that is physically removed from the strain sensing elements that resolve the force vector. The temperature compensation sensors disclosed above cannot be mounted in direct contact with the fiber optic sensor because they will interfere with the operation of and/or create a random error in the strain indications. Additional limitations of certain temperature compensation techniques may include added stiffness of the sensing structure (e.g. thermocouples), an increased profile of the catheter, or an increase in the complexity of the assembly.
The above-mentioned references also do not disclose ways to mitigate the effects of thermal gradients within the sensor tip. Thermal gradients differ from bulk temperature changes in that they produce a range of temperatures that are present simultaneously within an assembly, even when the assembly is in thermal equilibrium. Thermal gradients commonly occur in catheter assemblies that deliver energy to a target area, such as RF ablation electrodes. The end effectors in these systems may require, for example, the delivery of an irrigation fluid that is substantially cooler than the surrounding blood temperature in order to effectively cool the end effector. The catheter and force sensing assembly contained therein may therefore be in contact with both warmer and cooler components relative to the surrounding (body fluid) temperature, causing the temperature of the force sensing assembly to be non-uniform, even when the catheter is at thermal equilibrium.
It is therefore desirable is to provide a diagnostic and treatment apparatus, such as a catheter system, that permits computation of forces applied to a distal extremity of the catheter and is substantially insensitive to the thermal environment encountered during use of the catheter, such as exposure to body fluids and the presence of room-to-body temperature gradients, delivery of energy by an end effector and other sources of thermal gradients.